This invention relates to polysiloxane-modified epoxy resin compositions. In a specific aspect, the invention relates to molding powder formulations from such compositions, suitable for use as encapsulates for semiconductor devices, and articles made from such formulations.
Encapsulation of semiconductors and other components with an epoxy resin, is widely used in the electronics industry to provide physical integrity and environmental protection to electrical components.
Encapsulation is generally a process in which an electrical component, or an assembly of small discrete units, is cast with, or imbedded in, a molten film, sheath, or foam of an encapsulation composition, which is then subjected to conditions effective to solidify the composition. Once the encapsulation composition is solidified, the encapsulated component will have physical integrity and environmental protection. The most commonly used encapsulation technique is transfer molding. In this process, the component to be encapsulated is placed into the cavity of a mold and an encapsulation composition, liquefied by heat and pressure, is forced into the cavity where it solidifies to encapsulate the component.
An encapsulation composition must not soften during the normal usage of the encapsulated component, must not develop excessive internal stress during the encapsulation process, and must have minimal moisture gain. An encapsulation composition that softens during the normal usage of the encapsulated component will jeopardize the integrity of the encapsulated component. The softening of the encapsulation composition during the normal usage of the encapsulated component can be prevented by incorporating into the composition an encapsulation resin with a sufficiently high glass transition temperature (T.sub.g). Encapsulation compositions that develop high internal stress tend to break apart during normal usage. The trend in electronics technology is toward larger die sizes, increased capacity, smaller packages with higher pin counts and finer pitches, which in turn requires resins with low internal stress properties for very large scale integrated (VLSI) circuit chip encapsulation. Thus in encapsulation, as in many other epoxy applications, it is important that the composition used have low internal stress and high T.sub.g.
The internal stress (.sigma.) of a composition can be estimated by the following equation: EQU .sigma.=k.intg.E .alpha.dT
wherein E is the modulus of the material, k is a constant, T is temperature, and .alpha. is the coefficient of thermal expansion. Since the internal stress is proportional to both the modulus and the coefficient of thermal expansion, this suggests that the internal stress can be lowered by reducing the coefficient of thermal expansion, the modulus or both.
The common approach towards lowering internal stress has been to modify the epoxy resin with an elastomer. However, with increasing amounts of elastomer modifier, with filled or unfilled material, the modulus tends to decrease while the coefficient of thermal expansion tends to increase. Thus the reduction in internal stress is marginal due to the opposing trends of the modulus and the coefficient of thermal expansion. In addition, the T.sub.g, which for encapsulation must generally be at least about 150.degree. C., will many times vary proportionally with the modulus. Sometimes the lowering of the modulus results in too great a reduction in the T.sub.g for encapsulation application.
It is therefore an object of the invention to provide an epoxy-based encapsulation composition. In one embodiment, it is an object of the invention to provide a low internal stress epoxy composition without significantly sacrificing T.sub.g. It is another object to provide molding powders and articles made therefrom.